That doughnut did the same in mine. Couldn't find a readily available replacement so had to move onto the later type. This for me meant sourcing a new output shaft for my box to replace the existing one as someone had cut the spigot off required to fit the new type doughnut to fit the old.

Then a later spec front section of the rear 4x4 prop sourced and fitted and it now means I have the newer doughnut type which is readily available. I think I put some photos up of the two types and changes I made on my thread if that's any help to you.

If your box doesn't have the right output shaft, instead of changing the output shaft (which was nearly impossible to source) or getting a new box I believe GEARS (I think) have made a handy adaptor where all you do is swap the drive flange but for their type which has a spigot machined onto the new nut. I would have done this if I had known before I got the output shaft replaced.

Worth a little research as it's a very simple fix for the box!

Hope that helps